Generally, in the technical field of bolt tightening, it is sometimes necessary to provide controlled shear bolts, for various technical reasons, for example, to limit the tightening torque of the bolt, to prevent its removal, or even to prevent the bolt head from protruding from the surface of the part in which it is installed, once tightening is complete.
Bolts having several controlled shear zones are sometimes useful, for example, when the bolt is required to be broken as closely as possible to the surface of the part in which it is installed, in relation to the depth of its installation. It is equally important that the maximum amount of female thread available on the part is in contact with the male thread of the bolt, in order to distribute the tightening forces better on the female thread and, consequently, on the part.
Such bolts are used, for example, to secure conductors to electrical connectors, for example, sleeve-type electrical connectors, designed to connect several conductors between them.
They are also used as clamping bolts to maintain electrical cables on anchoring or hanging clips disposed on electricity poles.
There are other bolts known from document EP 0819222 having several controlled shear zones. These controlled shear zones are realized by a plurality of grooves provided along the length of the threaded body of the bolt, and wherein the depth decreases towards the head of the bolt, so that the torque required to shear the bolt is greater around the head of the bolt than around its opposing free end. Therefore, there is a large number of shear “stages”, so as to maximize the number of male threads engaged in the female thread, to distribute the tightening forces better. Moreover, these “stages” are used to minimize the protrusion of the bolt following installation.
In practice, this type of bolt poses several problems: on the one hand, these bolts are fragile, they may break during transportation, or when being handled during installation, and on the other hand, the threaded surface is reduced in so far as the grooves are made in the threaded portion of the bolt. The distribution of forces is thus deteriorated.
There are also other bolt assemblies known from document EP 1376764, having several controlled shear zones, without any loss of threaded surface. This involves a bolt assembly comprising a bolt body having a threaded portion at a free end from which protrudes a central shank having a plurality of controlled shear zones. At the free end of the shank, a head designed to be able to cooperate with a rotary drive tool has been provided to transmit rotary motion to the shank of the bolt body and, consequently, to the threaded portion. A plurality of threaded rings are disposed around the central shank. The rings are locked against rotation in relation to the shank due to its non-circular form and the complementary form of the rings. The thread of the rings are designed to merge with the threaded portion of the bolt body and are of a thickness such that the contact areas between two consecutive rings correspond to a shear zone found at the same level on the shank.
A major problem with these two types of controlled shear bolt assemblies resides in the fact that the shear zones must be designed so that the torque required to shear the bolt is greater around the head of the bolt than around its opposing free end.
In other words, the bolts must be dimensioned to break more easily when they have a low installation depth than when they have a high installation depth.
In practice, this means that, if a small element requires tightening, the bolt needs to be screwed in deeper than for a large element. Consequently, in the context of the two aforementioned documents, this means that small elements are tightened more than large elements.
Therefore, for example, in the field of tightening electrical cables in a connector, in order to meet the electrical and mechanical standards the tightening torques for these bolts will necessarily be oversized on some sections, because the tightening torque for the cable with the largest allowable section for a given connector will be the lowest shear torque for the whole of the bolt. The tightening torques for smaller cable sections will thus necessarily be higher, due to the design of these bolts.
In these circumstances, the tightening torque is sometimes so high in small-section cables that it can occur that a cable having a small diameter may break even before the bolt shears.
Another problem common to these two technical solutions resides in the fact that, when the shear zones are close to each other, the designer must provide low thickness shear zones, and which therefore have a low radius of curvature around the smallest diameter of the groove forming the shear zone.
In practice, it has been noted that low radii of curvature around the shear zones were locations of high stress concentrations and resulted in a less stable shear. These shear zones therefore have a wide shear torque tolerance interval. With regard to the user, this means a difference in shear torques between two identical bolts for an identical application.